1. Field of the Invention
The present invention is related to improved horizontal emitting, vertical emitting, beam shaped, distributed feedback (DFB) lasers fabricated by growth over a patterned substrate with multiple overgrowth.
2. Description of the Related Art
A laser diode is a semiconductor device that emits light in a stimulated manner when electrically biased in the forward direction, the feedback required to reach auto oscillation being provided by mirrors. The mirrors are usually obtained by cleaved facets or mirror-coated facets, in the case of horizontal in-plane lasers, or by dielectric multilayer mirrors, in the case of vertical cavity surface emitting lasers (VCSELs). Some lasers incorporate a dispersive element, such as a grating providing distributed feedback (DFB), to achieve single mode emission.
Lasers are comprised of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a pn junction. When forward biased, electrons are injected into the junction from the n-region and holes are injected from the p-region, usually in a thin emitting layer called a quantum well. The electrons and holes release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, depends on the bandgap energy of the materials forming the pn junction.
As semiconductor materials have improved, the efficiency of semiconductor devices has also improved, and new wavelength ranges have been achievable. Gallium Nitride (GaN) based lasers are probably the most promising in a variety of applications. GaN provides efficient emission from the ultraviolet (UV) to amber spectrum, when alloyed with varying concentrates of indium (In), for example.
Unfortunately, besides huge materials challenges, nitride lasers are difficult to manufacture. The cleaved facets have a low reflectivity due to the low refractive index of nitrides. Moreover, cleavage is hard to achieve because of the sapphire substrate. To obtain good nitride lasers, it is therefore desirable to obtain good mirrors with a planar fabrication technique.
Other functions are also desirable, such as vertical emission, high power, single mode emission, etc. VCSELS are, however, extremely hard to obtain, as the high quality mirrors required prove extremely difficult to manufacture, and VCSEL action has not yet been obtained in GaN based lasers.
U.S. Utility application Ser. No. 11/067,957, which is set forth above and incorporated by reference herein, is aimed at solving these challenges by using device growth techniques over patterned substrates, wherein the patterns have the functions of mirrors, light extractors, and beam shaping elements.
However, a serious problem arises with the structures described in U.S. Utility application Ser. No. 11/067,957. In semiconductor lasers, optical confining layers are used to confine the laser mode away from the metal top electrode. Such confining layers must have an index of refraction smaller than that of the active layer, and therefore somewhat repel the optical wave into the active region, thus leading to a weaker interaction between the laser wave and the electrode.
This solution cannot be used in the structures described in U.S. Utility application Ser. No. 11/067,957. The confining power of the typical materials used to confine light in a nitride materials system, e.g. Aluminum Gallium Nitride (AlGaN), is not strong enough, because the typical materials do not have a high enough refractive index contrast with the active material GaN. Then, if a thick top confining layer is used to separate the optical wave from the metal electrode, the optical wave will be poorly confined in the active material region, meaning that it will only weakly interact with the pattern of the LEO mask layer.
Some classic solutions can be used to improve the situation, for instance, by using different thicknesses of confining layers under the top electrode and in the region of the mirror patterns. However such solutions introduce new fabrication steps, and lead to some degree of radiation loss, for instance, at the junction between thinner and thicker confining layers.
There is a need, then, for improved methods for fabricating horizontal emitting, vertical emitting, beam shaped, distributed feedback (DFB) lasers, by growth over a patterned substrate, with multiple overgrowth. The present invention satisfies this need.